Band 3 is the major protein of the human erythrocyte membrane and immunologically cross-reactive species have been identified also in nonerythroid cells. The long term goal of this laboratory is to more thoroughly characterize the structure and function of band 3 in red blood cells and to establish the existence of homologous proteins with a similar structure and function in nonerythroid cells. The three major objectives of the current proposal are: i) to crystallize the isolated cytoplasmic domain of band 3 and solve its crystal structure, ii) to locate the subsites on the cytoplasmic domain of band 3 which bind the major cytoskeletal anchoring proteins, ankyrin and band 4.1, and then to investigate the regulation of these interactions, and iii) to screen nonerythroid cells for band 3-like proteins and then to isolate and characterize one or more of these homologous polypeptides. Because the function and/or malfunction of band 3 is reported to be involved in i) senescent red cell removal, ii) several types of hereditary spherocytosis, iii) invasion of the malarial parasite into erythrocytes, iv) attachment of Heinz bodies to the red cell membrane and the consequent hemolytic anemia, v) CO2 transport from the tissues to the lungs as HCO3-, and vi) attachment of the cytoskeleton to the membrane, a more thorough understanding of this protein's structure and function should be relevant to several health fields. The experimental protocols for each of the three main goals listed above involve biochemical techniques which are quite standard for the areas studied. These techniques include: protein crystallization, x-ray diffraction, affinity chromatography, immunoblotting, immunofluorescence microscopy, proteolytic dissection, and simple protein binding assays.